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THIS SNOWPACK SUMMARY EXPIRED ON January 1, 2017 @ 10:30 amSnowpack Summary published on December 31, 2016 @ 5:30 pm

Issued by
Ben Bernall - Kootenai National Forest

Avalanche Character 1: Persistent Slab

Persistent Slab avalanches can be triggered days to weeks after the last storm. They often propagate across and beyond terrain features that would otherwise confine Wind and Storm Slab avalanches. In some cases they can be triggered remotely, from low-angle terrain or adjacent slopes. Give yourself a wide safety buffer to address the uncertainty.

Multiple weaknesses and slabs are buried within the snowpack at this time. Snowpack analysis and stability tests conducted revealed that these weaknesses were bonding well and showing a low likelihood of reacting to human triggers at this time.

Bottom Line: Weaknesses in the snowpack have responded well to multiple days of moderate temperatures and no additional loading since Tuesday night. There are still weaknesses however and backcountry users should assess each slope individually and approach steep slopes with caution. Snowmobilers riding in the Purcell Range of the Kootenai may find these layers to be more responsive to human triggering due to the thinner snowpack and resulting weaknesses.

Avalanche Character 2: Wind Slab

Wind Slab avalanches release naturally during wind events and can be triggered for up to a week after a wind event. They form in lee and cross-loaded terrain features. Avoid them by sticking to wind sheltered or wind scoured areas.

Earlier this week the mountains of the Kootenai National Forest were loaded with an additional foot of snow and high winds. This created some potentially dangerous conditions near exposed ridgelines and on steep windloaded slopes. Todays travels revealed that these slabs are decreasing in reactivity as ample time has allowed the new snow to settle and bond.

Bottom Line: This hazard is reducing in sensitivity. Backcountry skiers and snowmobilers pushing their way into very high exposed terrain will be the most likely to encounter this hazard. Use caution in exposed, high elevation terrain and look for signs of wind loading before committing to a steep slope.

Snowpack Discussion

Today we traveled into the North Fork Keeler country and toured out to Burnt and Survey Mountain. Extended column tests failed with moderate force and no propagation. Failures occured underneath 12" of new snow that fell earlier this week, this failure was on the new snow/old snow interface. Compression tests revealed a weakness 21" below the surface. Failure occured on a buried surface hoarlayer with moderate force (CTM 18 Quality 2 shear). This layer was not at all reactive in Extended Column tests but it is worth paying attention to as it may be more reactive on steep slopes with a thinner snowpack.

DISCUSSION: Forecast models continue to indicate the potential for
some light snow to develop late this morning into the afternoon
across mainly northwest Montana. Elsewhere, some breaks in the
clouds are anticipated with temperatures slightly cooler than
Friday.

A big change in the weather pattern will begin late tonight as an
arctic front moves across the Continental Divide from Canada and
collides with an upper level weather system from the northwest.
Widespread snow will develop, with the focus of the best
accumulations expected through central Idaho and along the
Bitterroot Mountain chain. In addition, gusty northeast and east
winds will develop as the arctic airmass moves into the region.
Gusty winds up to 40 mph will be possible through favored canyons,
particularly through northwest Montana. Much colder temperatures
will settle into the region by early next week.

This advisory applies only to backcountry areas outside established ski area boundaries. This advisory describes general avalanche conditions and local variations always occur. This advisory expires midnight on the posted day unless otherwise noted. The information in this advisory is provided by the USDA Forest Service who is solely responsible for its content.